[Forward port 15.0] #saveUnpublished workflow method should only be available if there is not another user editing the draft

Forward port CMS-13401 to 15.0 now using the SXCML changes of CMS-13400

If the current user is editing the draft and makes changes to the unpublished (s)he should be allowed to

1. still obtainEditableInstance (again)
2. invoke saveUnpublished

The user should still be allowed to 'obtainEditableInstance' since this hint can be used to find out is someone else is editing already (note this already works correctly)

If another user is editing the document and the current user makes changes to the unpublished, then

1. obtainEditableInstance should not be allowed
2. invoke saveUnpublished should not be allowed